1. Field of the Invention
The invention relates to telephone or data systems and more particularly to an improved method and apparatus for automatically correcting errors in the cyclic routing memory of a four-wire time division multiplexed (TDM) exchange.
2. Prior Art
Time division multiplexed exchanges usually comprise a switching part and a control part or means.
The switching part is built up in three parts: an incoming device or means, a switching device or element, and an outgoing device or means.
Under the control of the control part, the speech information of each channel to be switched is transmitted from the incoming device through the switching device to the outgoing device. The switching is done on a time-by-time basis in this sense that a part of the speech information of one line is switched through the exchange, then a part of the speech information of a second line is switched through the exchange, etc. till a part of every line has been switched and the cycle can start from the beginning again. This switching is done under control of a cyclic routing memory in the control part. Such a cyclic routing memory which is incorporated in the control part usually consists of a number of rotating memories with a fixed number of lines. Every line usually contains the data to establish one switching path through the exchange. If the information in the cyclic routing memory is completely lost, no switching paths can be established anymore; but if there is a failure in the data in the cyclic routing memory on only line, it is impossible to establish the switching path which is controlled by the data on that line.
The two most frequent failures are:
1. loss of data caused by a disturbance; PA1 2. a physical fault in the cyclic routing memory which is a result of e.g. burn-out.
As the loss of data in such a cyclic routing memory is known in this art as a serious problem, roughly two solutions have been proposed. The first known solution is to double the cyclic routing memory; this gives the best solution in case of a physical defect. The second solution is to use a background store for storing the double data, which will do in most of the cases.
Both the above mentioned solutions have the same disadvantage that the data have to be stored twice and this will double the datatraffic on the databus that interconnects the source of the data and the operational cyclic routing memory, and interconnects the source of the data and second store or spare routing cyclic memory.
All the above is well known to those skilled in the art and the description of some of such TDM exchanges are found in the U.S. Pats. to Sternung, No. 3,458,659 issued July 29, 1969; to Texier et al, No. 3,952,162 issued Apr. 20, 1976; to Ghisler, No. 3,974,340 issued Aug. 10, 1976; and to Regnier et al, No. 3,920,914 issued Nov. 18, 1975; all those described exchanges being of the time-space-time type. The exchange contemplated herein, however, is of the time-space type. A switching device of this type has been described in the magazine "Het PTT-bedrijf", Vol. 18, No. 1, April 1972, Pages 73 through 79.